The invention relates generally to micro pin hybrid interconnect arrays and more particularly to a micro pin hybrid interconnect array structure and method for creating the structure that minimizes coefficient of thermal expansion (CTE) mismatches between a crystal and the substrate to which it is attached.
Cadmium zinc telluride (CZT) and cadmium telluride (CdTe) crystals are extremely brittle materials and pose great difficulties in packaging, especially for large and thin crystals. Recent discoveries have shown that micro-cracks form in the CZT adjacent to the interconnect produced using a low temperature conductive epoxy adhesive. The CTE mismatch between a crystal and the substrate induces substantial strain in the interconnect to crystal interface, causing severe reliability problems, especially when using organic substrates that have a CTE of 11 ppm/C or greater. The greatest strain often occurs when the parts are cured (if using an adhesive process) or reflowed (if using a solder process).
Substrate materials are typically selected with a CTE matched as closely as possible to the crystal to minimize the global strain generated during thermal cycling, as the substrate and crystal expand and contract. Alumina ceramic substrates are usually chosen for use with CZT detectors.
Further analysis has indicated that micro-cracks can form in CZT crystals mounted on ceramic substrates with conductive epoxy interconnects, as a result of local CTE mismatches between the conductive adhesive and CZT, within a single interconnection point. Conductive epoxies are known to have CTEs much higher than CZT. Also, when the temperature exceeds the glass transition temperature (Tg), of the epoxy, the CTE dramatically increases to values typically 4-5 times greater than the CTE below Tg.
Indium bump bonding, conductive adhesives and low temperature solder are currently used to attach CZT detector crystal substrates. Very little work has been done on the reliability of packages using these crystals; and the work done to date appears to be very limited when compared with industry standard packages. Thin (<5 mm) CZT and CdTe crystals are notoriously difficult to package without damage. CZT detectors are typically exposed to only the mildest reliability testing because they are known to be so fragile. It is well known that a 0.5 mm thick CdTe crystal can easily crack, even when carefully handled. The likelihood for cracking is very high when this type of crystal is attached to a high CTE interposer, such as an organic interposer.
It would therefore be advantageous to provide a structure for interconnecting CZT and/or CdTe to an interposer that offers low local strain and that can accommodate conventional attachment methods available today to manufacturers.